The invention relates to the optical projection of images, and in particular to an optical apparatus for embedding updateable hidden information in a projected image.
It is known that photographs or motion picture images (hereinafter referred to as source images) can have a visible message contained within the image area (for example: bar codes or date/time stamps). It is also known that numerous techniques exist to embed a hidden message within the image area of a source image. The hidden message may take on different forms, but in many techniques, the hidden message is a two-dimensional image that represents binary data or an icon (such as a copyright notice or a company logo). Such an image is hereafter referred to in the art as a data image. Applications for embedded data images include copyright protection and watermarking. A specific application in the motion picture industry is the detection and tracking of pirated movies that have been copied with video camcorders.
U.S. Pat. Nos. 6,018,374 and 5,959,717 relate primarily to film piracy in the motion picture industry. These patents disclose means for embedding data images in motion pictures by using combinations of two separate projection systems.
U.S. Pat. No. 6,018,374 discloses the simultaneous projection of the source image in the visible spectrum and a focused or unfocused data image in the infrared spectrum. The infrared data image would not be visible to the audience in the theater, but it would appear in a recording of the visual image made by a would-be thief using a video camcorder. Unfortunately, the deterrence to piracy can be overcome simply by using an infrared filter in front of the video camcorder to remove the unwanted data image.
U.S. Pat. No. 5,959,717 involves the combination of a film projector and an electronic video projector for showing a single composite motion picture. In one embodiment of this invention, hidden images or messages are embedded in motion pictures in the following way. A sub-area of the motion picture provided by the film projector is omitted or modified. The sub-area is a substantially smaller sub-area of the screen. The electronic video projector is coupled to a video source and is used to provide the image content of the omitted or modified sub-area on the projected film image. For example, the sub-area may include an alert message or a symbol that is camouflaged by the output of the video projector. When the composite motion picture due to both the film projector and the video projector are displayed, the hidden message is not visible to the audience. This invention improves copy protection of the film because a thief would have to steal both components in order to display a complete motion picture. However, once displayed as a complete motion picture, the hidden message is not visible, and consequently piracy of the motion picture in the theatre using a video camcorder is unaffected.
Other prior art techniques for embedding data images involve considerable computer processing. In these techniques, the data image is combined with the source image in such a way that it is very difficult to separate them without special knowledge (such as a secret key). Typically in these techniques, a source image in which a data image is to be embedded must first be converted to a digital file. Embedding is accomplished by computationally modifying the digital source file, followed by re-printing of the modified file. In many instances, however, it is not convenient or possible to process the source image in the digital domain, and it is necessary to embed the hidden data image by analog (i.e., optical) methods. For example, a conventional film-based movie projection system does not use a digital source file, and the embedding of a data image at the time of projection requires an optical method. Likewise, an optical method is required when embedding a data image during the manufacture of photographic film or paper or when producing photographic prints with an optical photographic printer.
U.S. Pat. No. 5,859,920 issued Jan. 12, 1999 to Daly et al. is a prior art technique for embedding digital data in images that can be implemented either in the digital or analog domain. In addition to the possibility for optical implementation, this technique has several other key advantages with respect to other known prior art methods, especially the techniques mentioned above in U.S. Pat. Nos. 6,018,374 and 5,959,717. These additional advantages include:
1) no visible distortion of the source image, yet the hidden message can be retrieved by suitable image processing;
2) the embedded data is not easily corrupted by source image content or defects; and
3) the embedded data is not lost when the image is cropped, rotated, resized or filtered.
The patent by Daly et al. discloses a method of embedding digital data that includes the steps of:
a) generating a data image from the digital data;
b) convolving the data image with an encoding carrier image to produce a spatially dispersed (also known as a phase dispersed or frequency dispersed) data image; and
c) adding the spatially dispersed data image to the source image to produce a source image containing the embedded data image.
The spatially dispersed data image represents a watermark pattern that is added to the source image. The data is recovered from the image by:
a) cross correlating the source image containing the embedded data image with a decoding carrier image to recover the data image; and
b) extracting the digital data from the recovered data image.
Provisions for an analog implementation of this method are also disclosed in U.S. Pat. No. 5,859,920. Although not specified in detail, the proposed analog method involves creation of xe2x80x9coptical versionsxe2x80x9d of the data and encoding carrier images. Convolution of these optical versions xe2x80x9c . . . may be performed optically using well known optical convolution techniques . . . xe2x80x9d. The resultant spatially dispersed data image is then projected onto photographic film, photographic paper, or a theater screen along with the source image.
However, no provisions are made in the patent by Daly et al. to permit real-time modification of the data image. The ability to update the data image can be advantageous in many applications. For example, in motion picture projection systems, the data image may include date/time and theater/screen information for a particular showing of a movie. This information can later be extracted from a pirated video to determine the source of the illegal copy. Obviously, it is necessary to update this information prior to each showing of the movie. Moreover, it may be desirable to change the data image during the movie showing to allow the time stamp or other information to be updated at a specified interval, possibly with each projected frame.
It may also be advantageous to permit real-time modification of the encoding carrier image. Changing the encoding carrier image will cause the spatially dispersed data image (i.e., the watermark pattern that is added to the source image) to change, which can be particularly beneficial in a movie projection system. It is well known that using a fixed watermark pattern for all frames in a motion picture makes the watermark pattern vulnerable to removal using relatively simple image processing techniques. In addition, although the watermark pattern is usually embedded at very low signal amplitudes, a fixed watermark pattern combined with the changing source images of a movie sequence can produce a highly visible pattern that would be objectionable to a person viewing the movie. By changing the encoding carrier image with each projected frame or at a specified interval, these problems can be overcome.
Thus, there is need for a technique for embedding a data image in source images that:
1) incorporates all of the advantages of the spatial (or frequency or phase) dispersion method disclosed in prior art,
2) can be implemented entirely in the optical domain, and
3) allows the data image and/or encoding carrier image to be updated in real-time.
The present invention is directed to overcoming the problems set forth above. Briefly summarized, according to one aspect of the present invention, an apparatus for optically embedding an updateable hidden data includes:
a) optical masks bearing a data image and an encoding carrier image either or both of which may be updated in real-time;
b) optics for convolving, the data image with the encoding carrier image to produce a spatially dispersed data image; and
c) optical beam combiner for combining the spatially dispersed data image with the source image to produce a source image containing the embedded hidden data.
The source image containing the embedded data can then be projected onto a motion picture theatre screen, photographic film in a camera, photographic paper in a photographic printer, or the sensor in a solid state imaging device, to name but a few.
The embedded data is recovered from the image by converting the source image containing the embedded data to a digital file and using the digital means as outlined above in U.S. Pat. No. 5,859,920. Alternatively, an optical mask bearing the source image containing the embedded data can be generated and optical cross-correlation with an optical mask bearing the encoding carrier image can be used to recover the hidden data image by optical means. The optical mask bearing the data-embedded source image is generally static during the recovery process; however, the mask bearing the encoding carrier image could be either static or electronically updateable.